The invention relates to apparatus for treating running webs of paper, textile material or the like, and more particularly to improvements in rolls which can be used in calenders and other types of machines and have external surfaces defined by stacks of absorbent discs.
It is known to assemble washer-like absorbent discs into a stack and to mount such discs on a core to form a roll which can be used with one or more additional rolls to define one or more nips for the passage of a running web of a material which contains a surplus of moisture or whose moisture content is to be increased. Reference may be had, for example, to German Pat. No. 498,397, to German Offenlegungsschrift No. 31 08 747 or to British Pat. No. 725,762.
FIG. 1 of German Pat. No. 498,397 shows a roll wherein one of the end walls has a frustoconical surface which abuts the adjacent absorbent disc of the stack on the core and converts the neighboring discs into funnel-shaped bodies each of which has an inner and an outer frustoconical surface. The conicity of the surface on the end wall is approximately 160 degrees. The discs which are disposed substantially midway between the end walls are flat, and the conicity of the discs in the stack increases in a direction from the center toward the end wall which is shown in FIG. 1 of the patent. The density of discs in the regions immediately adjacent to the periphery of the core exceeds the density of such discs in the region of the external surface of the roll. This is desirable in order to ensure that the density of the stack of discs does not decrease due to a reduction of internal stresses as a result of prolonged use of the roll and after repeated grinding to refinish the external surface.
The roll which is disclosed in the German Offenlegungsschrift No. 31 08 747 comprises two flat end walls which flank a stack of large concentric discs alternating with smaller concentric discs. The larger-diameter discs are made of an absorbent fibrous material and are flat, i.e., their planes are normal to the axis of the core. The smaller-diameter discs ensure that the density of the composite stack of smaller- and larger-diameter discs between the two end walls is greater in the region around the periphery of the core. The roll which is disclosed in this publication serves to withdraw moisture, and the density of its larger-diameter discs at the periphery of the composite stack of discs is less than in a calender roll, namely only between 80.degree. and 95.degree. Shore A.
All of the known rolls exhibit the drawback that they undergo extensive and unpredictable deformation in response to the application of pressures which develop in the nip during passage of a web of textile material or the like. If the pressure (with reference to the hardness of the radially outermost portion of the roll) is sufficiently small, the roll undergoes a mere elastic deformation and is capable of restoring its cylindrical shape immediately downstream of the nip. However, if the pressure in the nip is rather pronounced (this is desirable in connection with the treatment of many types of web-like and other materials), the fulling action upon the roll in the region of the nip entails a densification of the discs with attendant increase of hardness of the roll in the region of its peripheral surface. As a rule, the nip pressure should not exceed 50 daNcm when the hardness of the radially outermost portions of the discs is about 90.degree. Shore A. However, if the hardness is about 80.degree. Shore A and the nip pressure remains in the range of 50 daN/cm, the hardness automatically increases, at an unpredictable rate, to about 90.degree. Shore A after the roll completes a relatively small number of revolutions (in the range of one or more thousand revolutions). The peripheral surface of the thus hardened stack of discs exhibits an undulate shape and develops cracks so that it cannot subject a running web to a predictable moisturizing, demoisturizing or other action. The only remedy is to grind the roll so as to reestablish a cylindrical external surface.